Endeavour astronauts replace faulty gyroscope

— -- Two astronauts eased a new positioning device into place on the outside of the International Space Station Monday, but looming over them was the possibility of a perilous unplanned spacewalk to fix a deep gouge in space shuttle Endeavour.

Astronauts Richard Mastracchio and Dafydd Williams inserted a device known as a gyroscope into a slot aboard the station after taking out a malfunctioning gyroscope that shut down last fall. After the two fastened the new one in place, Mission Control planned to turn on the gyroscope to see whether it worked.

"Good job, you guys," said their crewmate Tracy Caldwell, who was directing the spacewalk from inside the space station.

To get the new gyroscope into position, Williams, a Canadian, carried it in his arms while the station's robotic arm carried him to the right spot.

"I'm looking forward to riding after all this walking," Williams said. Later he called it "the most amazing ride I've ever had" and said the arm's motion was "incredibly smooth."

"That's because I ain't driving it," said his crew mate Charles Hobaugh from inside the station. "How can I take credit for that."

Unlike the gyroscope replacement, a spacewalk to repair the deep gash in Endeavour's belly would be both difficult and risky. Spacewalkers working on the orbiter's underside would not have handrails to grab, as they do when working on the space station. The belly is covered by heat-resistant tiles so brittle that they could be dented if brushed by a tool or a spacesuit helmet.

NASA managers will decide as early as today whether the spacewalkers need to repair the gash, which penetrates all the way through the tile layer to expose the stabilizing material beneath. Roughly three inches long and an inch deep, the gouge was made by a piece of foam insulation that broke free from the shuttle's fuel tank during Endeavour's liftoff Aug. 8.

A blow by a similar piece of debris led to the destruction of shuttle Columbia in 2003, killing all seven astronauts aboard. Since then NASA has made numerous improvements to the fuel tank to keep the foam in place, but the upgrade that would have prevented the loss of the foam that hit Endeavour won't be implemented until 2008.